I. Field of the Invention
This invention relates to a drive and specifically a drive and a method for operating a variable drive such as a variable-speed motor drive system.
II. Description of Prior Art
Variable-output drives traditionally are used in industry to provide variable electric power to AC motors. These same drives can be used in other applications not related specifically to motors but where a variable-output voltage or frequency is desired. Typical drives have an AC input power source and some type of conversion apparatus, usually using solid-state devices, for converting the fixed AC input voltage into a variable-voltage and/or variable-frequency output. One such type of drive is described in U.S. Pat. No. 5,625,545, which is incorporated herein by reference. That patent describes a power supply used as a drive which utilizes a number of power cells arranged to produce a three-phase AC output. Such multiple power cells in series can be utilized to provide higher voltage outputs than would be available with only a single cell. However, should a given cell in a series arrangement fail during operation, that current path can become open-circuited, rendering the entire drive inoperable. In many applications, especially industrial applications utilizing variable-speed drives, it is desirable to continue to operate in a reduced mode when a single cell fails. One method to do such a reduced operation is to short out the damaged or failed cell. This can then result in an unbalanced output, in that the leg having the shorted cell must operate at less than full voltage. Such operation can create imbalanced line conditions and be damaging to load elements. Therefore, when a cell fails in a series arrangement, it is usually desirable to permit operation at a reduced voltage or speed by shunting out an equal number of cells in each of the other legs. While the output from such a drive having shunted cells in each leg is reduced, it does permit the drive to operate at a reduced voltage or speed. Often times such reduced operation is highly desirable so that the process being controlled by the drive can continue until it is convenient to shut down for repairs. In addition, it may be desirable to operate the drive at a reduced-voltage condition so as to maintain a desired minimum level of production. Typically, when a drive having multiple converters arranged in series has a failure, the output voltage and power available is reduced proportional to the ratio of the number of unfailed cells in a given leg divided by the number of total cells in such leg. As an example, if one of the cells in the aforesaid patent, such as power cell A.sub.1 were to fail, typically power cells A.sub.1, B.sub.1 and C.sub.1 would be shunted by appropriate bypass circuitry. Such circuitry is shown in FIG. 9 of the U.S. Pat. No. 5,625,545. When operating under such bypass condition, the output of the three-phase drive at references 22, 25 and 24 is proportionally reduced by one-third. In such an operating condition, the output voltage available to the motor would only be 66% of the full voltage. In such a condition, the motor would only be able to reach approximately two-thirds of its rated speed. In many applications being able to reach even 66% of rated speed in a failed cell mode is desirable. However, it still represents a substantial reduction in the output speed and power available from the drive. When multiple power cells or converters are used in series, the total power delivered to the load is usually supplied generally equally by all the power cells. Thus in the example above when one of nine cells fails, 89% of rated power should still be available. By shunting out or bypassing two operational power cells such as B.sub.1 and C.sub.1 in the example, these power cells can no longer contribute to the output power available to the load. It would be very desirable, then, to optimize the output voltage and power available to the load during a failed cell condition. The invention provides for an apparatus and a method that permits operation during a failed mode at a power output that utilizes all of the cells or converters that are operational.